This invention relates to an electrode substrate for use in display devices. More particularly, the invention relates to an electrode substrate having a builtup structure capable of mitigating or relaxing an internal stress involved therein and also to a method for making such an electrode substrate.
Recently, there have been extensively developed display devices which have a flat panel structure made of a pair of electrode substrates mutually bonded together. A typical structure of an active matrix-type liquid crystal display device is shown in FIG. 8. As shown, the liquid crystal display device has a flat panel structure having a pair of electrode substrates bonded through a given gap therebetween. For convenience' sake, one of the electrode substrates is called a drive substrate 101 and the other electrode substrate is called a counter substrate 102. A liquid crystal 103 is provided between both substrates 101 and 102. The drive substrate 101 has on the inner surface thereof scanning lines 104 and signal lines 105 arranged as intersecting in the form of matrices. Pixel electrodes 106 are formed at individual intersections. Each pixel electrode 106 is made of a transparent conductive film such as ITO (composite oxide of indium and tin) which has been patterned in a desired form. Thin film transistors (TFT) 107 for switching drive are formed corresponding to individual pixel electrodes 106. The drain electrode of each TFT 107 is connected to a corresponding pixel electrode 106 and the source electrode is connected to a corresponding signal line 105. The gate electrode is connected to a corresponding scanning line 104. On the other hand, the counter substrate 102 has on the inner surface thereof a counter electrode 108 and a color filter film 109 superposed as shown. The counter electrode 108 is similarly made such as of ITO and forms a pixel in combination with individual pixel electrode 106. The color filter film 109 is divided into segments of red-green-blue (RGB) primaries.
FIG. 9 is a schematic sectional view showing the structure of the counter electrode of FIG. 8. The counter electrode has a glass substrate 201 and a color filter film 202 formed on one side of the substrate 201. The color filter film 202 is divided into the segments of the RGB primaries with black masks 203 being formed at boundaries of the respective segments and each serving as a light-shielding region. A flattened film 204 made of a transparent resin layer is formed on the color filter film 202, on which a counter electrode 205 made of an ITO film is further formed. The ITO film may be patterned in a desired form, if necessary.
For the patterning of the ITO film, etching and resist releasing steps are necessary. In these steps, an acid or alkaline solution is used, which may sometimes result in swelling of the flattened film 204. If the flattened film 204 is swollen, the ITO film being patterned becomes irregular at edge portions thereof as penetrated, with the possibility that defects such as film breakage take place. To avoid this, an underlying layer such as an inorganic SiO.sub.2 film may be provided between the ITO film and the flattened film 204. However, the provision of the SiO.sub.2 film results in the formation of a two-layered inorganic film structure on the flattened film 204. This will present the problem that the counter electrode 205 suffers separation or cracking owing to the internal stress of the individual films.
It is the usual practice to prevent the separation or cracking by properly selecting film-forming conditions such that the internal stresses of the SiO.sub.2 film and the ITO film, respectively, become close to zero. Alternatively, when the SiO.sub.2 film is internally stressed in tension, for example, film-forming conditions for the ITO film are so selected as to have an internal stress of compression. With the ITO film, however, the film quality is greatly influenced by the film-forming conditions. For instance, where the ITO film is deposited by sputtering, there may not be obtained a film of a uniform quality depending on a slight difference in the condition of contamination or target. Accordingly, in an actual fabrication process, it is difficult to invariably keep optimum film-forming conditions of ITO, resulting in complicated working operations.
In view of the fabrication process and reliability, it is easy and convenient to form, on a glass substrate, an ITO film and then a color filter film in this order. However, such an arrangement as set out above has the color filter film made of a dielectric material between the ITO film and the liquid crystal. This is not advantageous in view of working performance of the device. Especially, with high-duty drive color liquid crystal display devices, it is essential to use a structure which includes an ITO film formed on a color filter film. The reasons for this are as follows. First, in a twist nematic mode or super-twist nematic mode, the anisotropy of dielectric constant of the liquid crystal (.DELTA..epsilon.) is positive, so that the electric capacitance of a liquid crystal pixel changes between the on and off voltages. Accordingly, with the builtup structure of the color filter film on the ITO film, the on/off voltage ratio to be applied to the liquid crystal becomes considerably worsened, bringing about a lowering of contrast. Second, if a color filter film is formed on an ITO film, a voltage drop takes place. This requires a drive voltage to be set at a higher level in order to compensate for the voltage drop.
The problems of the prior art have been set out with respect to the counter substrate of the active matrix-type liquid crystal display device. These problems are not inherent to the counter substrate alone but are common to electrode substrates for display devices which include builtup structures having a resin layer, an underlying layer and a conductive layer formed in this manner. More particularly, in the known electrode substrate structures, there arises the problem that these structures suffer separation or cracking owing to the internal stresses exerted on the underlying layer and the conductive layer.